They pool their money together and then rotate who it all goes to every month or some pet period.
Sounds like a scam run on the financially illiterate.
https://www.modernghana.com/news/772009/tontine-microcredit-...
Where they work is when each member of the group is known to each other and therefore would lose social status by defaulting on their obligations.
The benefit of these arrangements when run properly is that instead of saving $10 per month and after a year having the $120 to buy a productive asset (let's say a cow), they can get the $120 up front from the Tontine and pay the money back to the community group out of the earnings from the milk etc.
The persistance of these schemes everywhere indicates that this is a valuable means for the less well off to gain access to lump sum amounts for purchases without paying interest. Not everyone will be happy about the last part of course.
I play a survival game called Rust. Progress is meted out through the slow accumulation of Scrap, which is spent on learning technologies. People constantly kill each other, and those who are ahead on tech tend to accrue advantages in combat. There is no "Bank", and no guarantee of the ability to set up a minimum viable base to store your resources. One of the most pathetic strategies to resort to on the busiest most crowded servers, when you're a few days behind, is to linger around the edges of a safe zone collecting small amounts of Scrap(dying every few minutes), literally play roulette with it at the in-game gambling system in the safe zone at 1:20 odds, and when you eventually win big, use your scrap to progress through the tech tree; It might be 1000 scrap to get your tech to where it needs to be to compete, and you might average 50 (w/ Std dev of 25) scrap between deaths, but you'll get there eventually in this way, whereas the non-gambler needs incredible luck (+38 standard deviations of success) to actually progress through this high price threshold.
A typical tanda removes the random chance element, but preserves the "Eventually you'll get there" element even while holding on to no money.